Scent of a Comet

Image taken on 26 September from a distance of 26.3 km from Comet Chury. The image shows the spectacular region of activity at the «neck» of the comet with ices sublimating and gases escaping from inside the comet. Credit: ESA/Rosetta/NAVCAM

How does a comet smell and what can it teach us about the emergence of our solar system? Since early August, the Rosetta Orbiter Sensor for Ion and Neutral Analysis (ROSINA) has been “sniffing” the fumes of the comet Churyumov-Gerasimenko with its two mass spectrometers to answer these very questions. The detected chemistry in the coma of the comet is surprisingly rich at more than 400 million kilometers from the Sun.

The smell of this comet is quite strong, with the odor of rotten eggs (hydrogen sulphide), a horse stable (ammonia) and the pungent, suffocating odor of formaldehyde. This is mixed with the faint, bitter, almond-like aroma of hydrogen cyanide. Add some whiff of alcohol (methanol) to this mixture paired with the vinegar like aroma of sulphur dioxide and a hint of the sweet aromatic scent of carbon disulphide, and you arrive at the smell of this particular comet.

While this certainly doesn’t make a very attractive perfume, keep in mind that the density of these molecules is still very low and that the main part of the coma is made up of sparkling water (water and carbon dioxide molecules) mixed with carbon monoxide. “This all makes a scientifically enormously interesting mixture in order to study the origin of our solar system material, the formation of our Earth and the origin of life”, says Kathrin Altwegg from the Center of Space and Habitability (CSH) of the University of Bern. “And after all, it seems like comet Churyumov was indeed attracted by comet Gerasimenko to form Churyumov-Gerasimenko. Even though its perfume may not be Chanel No 5, comets clearly have their own preferences”…

The idea was that at distances outside of 3 astronomical units (450 Mio km), the comet would mostly sublimate the very volatile molecules: carbon dioxide and carbon monoxide. However, apart from all these smelling ingredients, ROSINA is detecting many more molecules at large distances from the Sun which comes as quite a surprise. A quantitative analysis will show how the comet compares with other comets where data are available mostly from remote sensing. This comparison will reveal if Churyumov-Gerasimenko, being a Kuiper belt comet, differs from the better known Oort cloud comets. This will then shed light on the solar nebula from which our solar system emerged.